The lifeblood of a fortress, a smelter is used to smelt ores and melt existing metal objects into metal bars. The smelter is also where lignite and bituminous coal are processed into coke. A dwarf must have the "Furnace Operator" labor enabled to use the smelter, and all smelter tasks require a bar of fuel.

If all options in a smelter are red, there are three possible scenarios to explain why:

The workshop is being specified as a "Give" destination from a stockpile that does not possess all of the necessary materials (including fuel).

The smelter is located inside a burrow, while one of the necessary materials (including fuel) is not available in the burrow (this problem occurs even if no dwarves have been assigned to that burrowBug:434).